This invention relates to a method for welding aluminum plates for manufacturing a large spherical structure and to a large spherical structure manufactured by this method.
A ship-borne tank for liquified natural gas (LNG) may be manufactured by welding together large and thick aluminum plates and bending the resulting plate assemblies to form them to portions of the desired spherical LNG-tank. The same kind of tank may be used elsewhere for the storing of LNG and other substances. Because a large part of the manufacturing costs are welding costs, the plate blanks that are welded together should be as large as possible. Joining together this kind of large plate blanks and welded plate assemblies by further welding is demanding work, because all weld joints must be of high quality. Also the plates and the plate assemblies must be precise in shape and dimension for achieving a proper final result.
U.S. patent application Ser. No. 08/061,193, the disclosure of which is hereby incorporated by reference herein, discloses a method of manufacturing a large spherical LNG-tank by welding together several plane aluminum plates to form a planar plate assembly and subsequently forming the plate assembly in a mold to part-spherical configuration and using the formed plate assembly in the following stage of manufacture. However, during bending of the plate assembly in the mold, the plate assembly stretches somewhat, which makes it difficult to use modern high power MIG-welding equipment for welding in one pass per side because there are shape defects in the welding groove and at its root.
It is known also to manufacture spherical tanks of plates that are welded together to form larger plate assemblies by a so-called tractor type welding carriage. However, use of a tractor type welding carriage is difficult and time consuming, especially at start and finish of the welding operation. Known manufacturing methods using tractor type welding equipment for welding together plates that have been cut to a particular peripheral shape and/or bent to a desired configuration also require twice as many assembly stations. It is necessary to remove the weld reinforcement before forming in order to allow the inspections that are included in many methods and because of the forming itself. Special attention must be given to the automation of the work and to the dimensional precision of the final work.